1. Field of the Invention
This invention relates to an image display apparatus for storing image data and displaying, on demand, images represented by the image data, and more particularly to an image display apparatus of the kind arranged to display images by switching one image over to another with some picture effect imparted to pictures displayed.
2. Description of the Related Art
Image display apparatuses of varied kinds have been proposed. A method for forming transparent images conventionally employed in general for still image display apparatuses is described below as follows:
FIG. 1 schematically shows the arrangement of an image display apparatus adopting the conventional method. Referring to FIG. 1, image data is read out from a disk 21 in accordance with instructions from control means (MPU) 20. The image data read out is written into a first frame memory (FM0) 22 and a second frame memory (FM1) 23.
The image data written into the frame memories 22 and 23 is read out at a video display timing to be displayed on a CRT 24. To bring forth a picture effect, the value of a transparency parameter a which is stored for the picture effect in a bit mapping memory (.alpha.MEM) 25 is caused to change as desired within a vertical blanking period by the control means 20. With the value of the transparency parameter .alpha. thus changed, the values of the image data of the two frame memories 22 and 23 are mixed together by an effector (EFF) 26 before the images are displayed. The transparency parameter .alpha. is provided for controlling some image to become semitransparent so as to make other images visible through the semitransparent image and is set to have a decimal value which is variable between "0.0" and "1.0" including some intermediate values.
The effector 26 is composed of a first computing element (SUB) 27, a second computing element (MUL) 28 and a third computing element (ADD) 29 and is arranged to perform a process of mixing the image data read out from the two frame memories 22 and 23 within an arbitrary area and in an arbitrary ratio by carrying out the following computing operation on the image data read out from the frame memories 22 and 23. More specifically, with the output of the effector 26 assumed to be Y, the output of the effector 26 can be expressed as follows: EQU Y=.alpha. (FM1-FM0)+FM0=.alpha. FM1+(1-.alpha.) FM0
By changing the value of the transparency parameter a gradually from "0.0" to "1.0", the output Y of the effector 26 gradually varies from the output FM0 of the first frame memory 22 to the output FM1 of the second frame memory 23 in a mixed manner. However, the conventional system such as the one described above has presented the following problem:
In a case where a large number of display picture elements must be converted, the amount of information of the bit mapping memory 25 to be rewritten during the vertical blanking period in bringing forth a picture effect increases. The increased amount of information either cannot be adequately processed, if the control means 20 is of an ordinary capacity, or causes an increase in cost.
To solve this problem, it is conceivable to use a dual-port memory for the bit mapping memory 25 in such a way as to permit rewriting the contents of the bit mapping memory 25 even during the vertical blanking period. Such a solution, however, would lower the quality of the picture effect as it makes an area where change-over takes place for the picture effect discontinuous while the picture effect is in process.
FIG. 2 shows by way of example a case where the value of the transparency parameter is gradually varied from "0" to "1" in a concentric elliptic shape from the center of the bit mapping memory 25 to its peripheral part so as to bring forth an elliptic wipe effect, with the output of the first frame memory 22 being white and that of the second frame memory 23 black. As apparent from FIG. 2, the number of dots in the change-over area of the picture (edge part of an elliptic shape) increases and, thus, the amount of rewriting of the contents of the bit mapping memory 25 during the vertical blanking period greatly increases.